If you have a list which contains a SPFieldUser field (with multiple selection), you can add users too it with the following code:

using (SPSite site = new SPSite("http://site"))

{

using (SPWeb web = site.AllWebs["Web"])

    {

    SPList list = web.Lists["List"];

    SPListItem item = list.Items[0];

    SPFieldUserValueCollection values = (SPFieldUserValueCollection)item["Users"];

    SPUserCollection users = web.AllUsers;

    foreach (SPUser user in users)

    {

        values.Add(new SPFieldUserValue(web, user.ID, user.Name));

    }

    item["Users"] = values;

    item.Update();

    }

}

In this example the list "List" would contain a field with the name "Users", which takes users. All web users are added to the field "Users" of the first list item, which is then updated.